A mass flow meter (Coriolis mass flow meter) based on the operating principle that when a flow tube through which a fluid being measured flows is supported at an end or both ends thereof, and is caused to vibrate at the fulcrums in a direction vertical to the direction of flow of the fluid in the flow tube, a Coriolis force acting on the flow tube is proportional to the mass flow of the fluid is well known. The Coriolis mass flow meter can be roughly divided into two types; a curved-tube type and a straight-tube type.
The straight-tube Coriolis mass flow meter, when caused to vibrate in a direction vertical to the axis of the straight tube at the central part of the straight tube supported at both ends thereof, detects mass flow as a displacement difference of the straight tube caused by a Coriolis force between the supporting parts and a central part of the straight tube, that is, as a phase difference signal. Despite its simple, compact and sturdy construction, the straight-tube Coriolis mass flow meter cannot accomplish high detection sensitivity.
The curved-tube Coriolis mass flow meter, on the other hand, can detect mass flow with high sensitivity since it can select the optimum shape for making effective use of Coriolis forces. In addition, a construction of the curved-tube Coriolis mass flow meter in which two parallel curved tubes through which a fluid being measured flows. are provided to effectively drive the curved-tubes is also well known.
FIG. 10 is a schematic diagram of a conventional type of two parallel curved-tube Coriolis mass flow meter, as mentioned above. As shown in the figure, flow tubes 1 and 2 comprise two parallel curved tubes (U-shaped tubes), and are caused to resonate with each other in an opposite phase by a drive unit 15 comprising a coil and a magnet, disposed at the central part of the flow tubes 1 and 2. A pair of vibration sensors 16 and 17, each comprising a coil and a magnet, are disposed at symmetrical positions with respect to the mounting position of the drive unit 15 to detect a phase difference proportional to Coriolis forces. A fluid being measured flows from an external flow tube connected via a flange 18 on the inlet side into a tubular meter body 34, and is diverted 90 degrees by a deflector plate 35, and divided equally to the two flow tubes 1 and 2. The divided flows are then joined at the outlet side of the flow tubes 1 and 2, diverted 90 degrees by a deflector plate 36, and discharged to an external flow tube connected via a flange 19 on the outlet side. By causing the fluid being measured to flow equally in the two flow tubes 1 and 2, as described above, the natural frequencies of the two flow tubes 1 and 2 can be maintained equal despite differences in the type of the fluid or in temperature. This allows the flow tubes to be driven efficiently and stably. Thus, a Coriolis mass flow meter that is not affected by external vibration and temperature can be accomplished.
However, this conventional type of Coriolis mass flow meter using two parallel curved flow tubes has not been perfect in isolating external vibration.
As shown in the figure, base plates 27 and 28 are provided on the two flow tubes 1 and 2, serving as the first fulcrum of vibration, while the joint parts between the two flow tubes 1 and 2 and the meter body 34 act as the second fulcrum of vibration of the flow tubes; both constituting an important basis for the vibration of the entire tubes. In the conventional type of Coriolis mass flow meter, however, the second fulcrum has not been perfectly isolated from vibration transmitted from the outside. As a result, external vibration transmitted from the meter structures, casing, etc. has had adverse effects on the performance of the Coriolis mass flow meter.
Furthermore, since this type of Coriolis mass flow meter using flow tubes comprising two parallel curved tubes has in its construction a branching part on the inlet side of the fluid being measured and a confluence part on the outlet side, pressure loss or fluid clogging is apt to occur. This is particularly true when a highly viscous fluid or a perishable and easy-to-clog fluid, such as food, is involved.
Furthermore, this type of Coriolis mass flow meter is required to be of a low-cost and sturdy construction so that it is sufficiently reliable even in case of damage to flow tubes. The conventional type of Coriolis mass flow meter, however, has been short of the requirement.
The conventional type of Coriolis mass flow meter has not been designed taking into account the effects of higher-order vibration modes that are intrinsic to vibrating flow tubes.
A drive unit 15 for driving the flow tubes 1 and 2 comprising two parallel curved tubes at the central part thereof normally comprises a coil and a magnet. The coil of the drive unit is installed on any one of the two flow tubes 1 and 2, and the magnet thereof is on the other flow tube so that the two flow tubes 1 and 2 are caused to resonate at an opposite phase with each other. A pair of vibration sensors 16 and 17, each comprising a coil and a magnet, are disposed at symmetrical positions with respect to the mounting position of the drive unit 15 to detect a phase difference proportional to a Coriolis force. The coils and magnets of these sensors are also provided in such a manner that the coil is disposed on any one of the flow tubes, and the magnet on the other flow tube via fixtures.
In these drive unit 15 and the vibration sensors 16 and 17, only the coils require wiring, and the magnets require no wiring. As a result, the wiring has been provided only on the surface of the flow tube having the coil in the conventional type of Coriolis mass flow meter. The conventional type of Coriolis mass flow meter, however, has not necessarily taken into consideration the effects of the wiring on the vibration of the flow tube: the coils of the drive unit 15 and the vibration sensors 16 and 17 have been concentrated on any one flow tube. As a result, this causes the effects of the mass and tension of the wiring to be concentrated on only the flow tube on which the coil is installed, disturbing the balance of the two flow tubes and resulting in adverse effects on the performance of the Coriolis mass flow meter.
This invention is intended to overcome the problems of the Coriolis mass flow meter of a type using two parallel curved tubes. It is an object of this invention to provide a high-precision Coriolis mass flow meter that ensures high vibration stability by isolating vibration transmitted from the outside to the vibration fulcrums.
It is another object of this invention to reduce the effects of higher-order vibration modes of the flow tubes.
It is a further object of this invention to provide a Coriolis mass flow meter of such a construction that vibration is hard to be transmitted via an inlet passage, and to substantially reduce pressure loss at the branching part at the fluid inlet and at the confluence part at the fluid outlet.
It is a further object of this invention to provide a Coriolis mass flow meter of a low-cost, mechanically strong and reliable construction, and to accomplish high accuracy by improving the vibration balance between the two flow tubes.
It is a further object of this invention to provide a thin-walled pressure-resistant case that can withstand very high pressures by forming the pressure-resistant case integrally with the meter body and rounding all corners of the case.
It is still a further object of this invention to reduce the adverse effects on the performance of the Coriolis mass flow meter by dispersing the coils of the drive unit and a pair of sensors to the two flow tubes, while disturbing the effects of the wiring so as to maintain the balance between the two flow tubes.